1. Field of the Invention
This invention relates to a compound semiconductor structure and a process of making the same, and more particularly to a compound semiconductor structure having a graded composition or mole fraction adapted for reducing the source resistance in a hetero-junction field effect transistor and a process of making the same.
2. Description of the Related Art
Applications of compound semiconductors involve the hetero-junction field effect transistor (HJ-FET). An example of the prior art of HJ-FET is shown in FIG. 6.
An undoped GaAs layer 2 is formed on a semi-insulating GaAs substrate 1. An undoped Al.sub.0.3 Ga.sub.0.7 As layer 3 is formed on the undoped GaAs layer 2 to form a hetero-junction. Due to the difference in the electron affinity in the materials GaAs and Al.sub.0.3 Ga.sub.0.7 As, there are formed steps .DELTA.E.sub.c and .DELTA.E.sub.v in the conduction band energy and the valence band energy, respectively, at the Al.sub.0.3 Ga.sub.0.7 As/GaAs hetero-junction interface. A quantum potential well originating in the gap of the conduction bands, .DELTA.E.sub.c, is formed in the undoped GaAs layer 2 adjacent the hetero-junction interface and results in the yielding of a two dimensional electron gas (2DEG), that is supplied from the donor in n-Al.sub.0.3 Ga.sub.0.7 As layer, by means of ionizing. 2 DEG in its quantum potnntial well should contribute as a channel of the HJ-FET.
On the channel region constituted by the undoped Al.sub.0.3 Ga.sub.0.7 As layer 3 and the undoped GaAs layer 2, are formed graded mole fraction layers 22 and 23 of Al.sub.x Ga.sub.1-x As sandwiched between uniform composition layers 21 and 24 of undoped GaAs and n type doped Al.sub.0.3 Ga.sub.0.7 As. The graded mole fraction layers 22 and 23 have a composition distribution in which the Al mole fraction x of Al.sub.x Ga.sub.1-x As changes from 0 to 0.3 as the position goes from the upper surface of the layer 22 to the lower surface of the layer 23, as shown at right hand side portion of FIG. 6. Thus, with respect to the Al mole fraction, the graded composition layers 22 and 23 connect the undoped GaAs layer 21 and the undoped Al.sub.0.3 Ga.sub.0.7 As layer continuously.
The layers 21 and 22 disposed below the gate electrode 9 are undoped to prevent the excessive increase in the field intensity, which may lead to avalanche breakdown.
Electrons are supplied from the source electrode 8 to the channel region 2, 3 through the layers 21, 22, 23 and 24. Electrons are transported through the channel formed by the hetero-junction layers 2 and 3 in the form of a two dimensional electron gas, whose electron number could be controlled by the voltage of a gate electrode. And those are derived through the layers 21, 22, 23 and 24 to the drain electrode 10. The source and drain electrodes 8, 10 may be formed of Au/Ni/AuGe and makes an Ohmic contact with the alloy under the condition of 400.degree. C. for 2 minutes. The gate electrode 9 may be formed of Al and established using the usual lift-off process.
FIG. 7 shows a band diagram in the HJ-FET of FIG. 6 in a direction perpendicular to the substrate surface (i.e. in the depthwise direction). The left end, i.e. outer left, region represents the surface GaAs region 21. The flatness of the diagram in this region means the state under the no electric field. In the case of the flowing electrons, of course, the band diagram should be inclined to right hand side. The inner left region, next to the surface GaAs region 21, represents the graded composition regions 22 and 23. The region 22 is undoped for providing a high breakdown voltage, as described above.
When the source electrode 8 is biased slightly negatively with respect to the channel region 2, electrons will flow from the source 8 to the channel 2 with a low resistance. The inner right region represents the n-type Al.sub.0.3 Ga.sub.0.7 As region 24 and the undoped Al.sub.0.3 Ga.sub.0.7 As region 3. In the right-hand side of these region 24, 3, the bands are bent upward due to the ionization of donor in n-type Al.sub.0.3 Ga.sub.0.7 As region 24. The right end i.e. the outer right region, represents the undoped GaAs region 2. Due to the differences in the energy of the conduction band and the valence band with respect to the vacuum level, there are generated gaps .DELTA.E.sub.c and .DELTA.E.sub.v in the bottom of the conduction band E.sub.c and in the top of the valence band E.sub.v, respectively.
If the graded composition region 22 and 23 is not provided that is, Al mole fraction of x is 0.3 in the regions, similar gaps may be formed at the interface between the surface GaAs region 21 and the Al.sub.0.3 Ga.sub.0.7 As above said region in the case of no graded one. The graded composition region 22, 23 serves to connect the bottom of the conduction band E.sub.c continuously, thereby enhancing transport of the charge carrier, electrons. This structure is reported, for example, in IEEE Transactions on Electron Devices, Vol. ED-33, No. 5 (1986) pp. 601-607.
Conventionally, the graded composition layer as described was formed through crystal growth by the molecular beam epitaxy (MBE), by changing the temperature of an aluminum cell during the growth to vary the Al mole fraction x of Al.sub.x Ga.sub.1-x As from 0.3 to 0.0.
Generally, one of the features of the MBE epitaxial growth of a compound semiconductor crystal is that the vapor pressures of atoms, and hence the rate of crystal growth, are regulated by controlling the temperatures of molecular beam sources. In the MBE epitaxial growth of a compound semiconductor, the constituent atoms of the compound are fed in the form of vapor into an ultrahigh vacuum space from two or more molecular beam sources and are solidified, in the form of an epitaxial crystal, on a semiconductor substrate crystal placed on a temperature-controlled susceptor, wherein the axes of the grown crystal follow those of the substrate crystal. The composition or mole fraction of the crystal to be grown, for example, x of Ga.sub.x Al.sub.1-x As, is determined by setting each vapor pressure of the molecular beam sources, Ga, Al and As. The vapor pressure of each source only depends upon the cell temperature of above said source. Thus, for making a crystal having a graded mole fraction x, it is necessary to vary the cell temperature of source to be varied. Conventionally, for grading a mole fraction x from a higher value toward a lower value, natural cooling has been utilized by switching off the electric power source which is the heat source for the cell of the atom concerned. Vapor pressure changes exponentially with temperature. Stable temperature control (for example of the order of .+-.0.1.degree. C.) is required for providing an accurate vapor pressure. For realizing a smooth composition distribution, it is preferable to gradually change the temperature. Thus, natural cooling has been adopted. FIG. 3 represents the relationship between the Al mole fraction of an Al.sub.x Ga.sub.1-x As crystal and the film thickness of the crystal grown under natural cooling of the Al cell. Initially, the respective cell temperatures are set to grow Al.sub.0.3 Ga.sub.0.7 As. The heater of the Al cell is turned off to allow the Al cell to be cooled naturally. As the temperature of the Al cell decreases, the Al vapor pressure decreases, after the relation P.sub.Al .alpha. exp (-.DELTA.E.sub.Al /kT.sub.Al), where .DELTA.E.sub.Al is the activation energy of Al and T.sub.Al is the Al cell temperature, resulting in gradual decrease in the composition x of the grown Al.sub.x Ga.sub.1-x As layer. As the value of x becomes small, the variation becomes gentle.
When the quantity of As vapor is sufficient, the composition x of the grown layer Al.sub.x Ga.sub.1-x As is determined by the vapor pressures of aluminum P.sub.Al and gallium P.sub.Ga. The growth rates are set, for example, ##EQU1## Here, the well temperatures of Ga and Al are set at T.sub.Ga =983.degree. C. and T.sub.Al =1090.degree. C. The growth rate of Al.sub.0.3 Ga.sub.0.7 As is the sum of the growth rates of GaAs 1 .mu.m/hr, and of AlAs, 0.43 .mu.m/hr. Letting the growth rate of AlAs be y .mu.m/hr, the composition x will be ##EQU2## The growth rate of AlAs is ##EQU3## When the Al cell temperature is 1090.degree. C. (=1363.degree. K.) in the growth of Al.sub.0.3 Ga.sub.0.7 As, the growth rate of AlAs can be expressed as EQU y=0.429 exp [(-.DELTA.E.sub.Al /k) (1/T.sub.Al -1/1363)].
Ordinary natural cooling requires about 30 seconds for lowering the Al cell temperature T.sub.Al by 100 degrees. For allowing the composition x to change from 0.3 to 0.0, it is necessary to grow a graded layer of the order of 400 .ANG. thick.
The crystal growth by MBE as described above is discussed in K. Takahashi, "Molecular Beam Epitaxy Technique" (1984) published by Kogyo Chosakai.
This conventional technique utilizing the natural cooling is useful for varying the mole fraction of an epitaxial crystal within a thickness of several hundreds of nm. Because of a relatively long growth time, the temperature can be lowered sufficiently. Thus, it was possible to vary, for example, the Al mole fraction x of Al.sub.x Ga.sub.1-x As from x=0.3 to x=0. However, in forming a thin film of around several tens of nm, the change in mole fraction from x=0.3 to x=0 was difficult owing to a too small growth time to cause sufficient temperature lowering.
It is indeed possible, to forcedly control the cell temperature by heating, etc. For example in the growth of Al.sub.x Ga.sub.1-x As by MBE, when the temperature of the Al cell is raised, the pressure of Al vapor varies in accordance with the instantaneous temperature of the Al cell, and hence the growth rate of the Al.sub.x Ga.sub.1-x As crystal varies. In such cases, control of the film thickness and the doping concentration becomes very difficult. This is because rapid temperature changes in a practical cell is hardly monitored accurately even when a thermo-couple thermometer is disposed near the cell for detecting the temperature and effecting temperature control. Thus, reproducibility and stability have been problematic.
A kind of superlattice structure as shown in FIG. 4 was proposed. The film thickness A of one kind of layer containing the element (Al in this case) and the film thickness B of another kind of layer which does not contain the element (Al) are set equal to each other (A=B). The sum of the thicknesses of adjacent pair of films (A+B) is increased. In FIG. 4, there is shown the relationship between the Al mole fraction and the epitaxial crystal thickness (proportional to the time of growth). The solid line represents the Al mole fraction of the grown film and the broken line represents the average mole fraction. This technique is described in Electronics Letters, 21, pp. 882-884 (1985).
U.S. Pat. No. 4,620,206 to Ohta et al (corresponding to JP-A-60-28268) discles a negative resistance device formed with a superlattice. Typically, a superlattice layer C is formed between material A layer and material B layer. In the superlattice, one end portion contacting the material A layer has properties substantially identical to material B and the other end portion contacting the material B layer has properties substantially identical to material A. A negative resistance phenomenon is exhibited, using above said structure involving homogeneous materials A and B and super-lattice. Here, if material A is exchanged by material B, and also B by A, the negative resistance phenomenon can not be observed in principle.